Geometry Of Fault System Research Articles

The Multi‐Segment Complexity of the 2024 MW ${M}_{W}$ 7.5 Noto Peninsula Earthquake Governs Tsunami Generation

AbstractThe 1 January 2024, moment magnitude 7.5 Noto Peninsula earthquake ruptured in complex ways, challenging analysis of its tsunami generation. We present tsunami models informed by a 6‐subevent centroid moment tensor (CMT) model obtained through Bayesian inversion of teleseismic and strong motion data. We identify two distinct bilateral rupture episodes. Initial, onshore rupture toward the southwest is followed by delayed re‐nucleation at the hypocenter, likely aided by fault weakening, causing significant seafloor uplift to the northeast. We construct a complex multi‐fault uplift model, validated against geodetic observations, that aligns with known fault system geometries and is critical in modeling the observed tsunami. The simulations can explain tsunami wave amplitude, timing, and polarity of the leading wave, which are crucial for tsunami early warning. Upon comparison with alternative source models and analysis of 2000 multi‐CMT ensemble solutions, we highlight the importance of incorporating complex source effects for realistic tsunami simulations.

Segmentation geometry of strike-slip fault systems in slow-deforming regions: a proposed method and case study of the Yangsan Fault, South Korea

Segmentation geometry of strike-slip fault systems in slow-deforming regions: a proposed method and case study of the Yangsan Fault, South Korea

Experimental study on the rupture behaviors of orthogonal faults: effects of stress state and rupture initiation location

Summary Fault rupture dynamics is expected to be significantly affected by the geometry of fault system, especially for orthogonal faults. However, the rupture behaviors of orthogonal faults especially the coseismic interactions are far from fully understood. Here, we present experimental results from a series of laboratory earthquakes to elucidate the effect of the stress state and initiation location on the rupture behaviors of orthogonal faults. Our results reveal a phase diagram of rupture behaviors of orthogonal faults, which is collectively controlled by stress state and rupture initiation location. For events initiating from the main fault, the rupture cannot jump to the branch, which may be due to the clamping effect or the inhibited shear stress accumulation on the branch. On the contrary, events initiating from the branch can persistently trigger ruptures of the main fault. This difference highlights the directional effect associated with the rupture of orthogonal faults. Further, the rupture length of triggered ruptures on the main fault is controlled by the stress state of the fault system. With the increase of the ratio between the shear stress and normal stress, the rupture length of the main fault increases. Our results reproduce the rupture behaviors of orthogonal faults, which may provide insights into the rupture characteristics of natural earthquakes.

Characterization and Evolution of Seismic Sequences in the Normal Fault Environment of the Southern Apennines

AbstractThe use of seismic catalogs enhanced through advanced detection techniques improves the understanding of earthquake processes by illuminating the geometry and mechanics of fault systems. In this study, we performed accurate hypocentral locations, source parameters estimation and stress release modeling from catalogs of microseismic sequences nucleating in the complex normal fault system of the Southern Apennines (Italy). The application of advanced location techniques resulted in the relocation of ∼30% of the earthquakes in the enhanced catalogs, with hypocenters clearly identifying local patches on kilometer‐scale structures that feature consistent orientation with the main faults of the area. When mapping the stress change on the fault plane, the inter‐event distance compared to the size of the events suggests that the dominant triggering mechanism within the sequences is static stress transfer. The distribution of events is not isotropic but dominantly aligned along the dip direction. These slip‐dominated lineations could be associated with striations related to fault roughness and could map the boundary between locked and creeping domains in Apulian platform and basement.

Spatial migration of temporal earthquake clusters driven by the transfer of differential stress between neighbouring fault/shear-zone structures

Uncertainty concerning the processes responsible for slip-rate fluctuations associated with temporal clustering of surface faulting earthquakes is a fundamental, unresolved issue in tectonics, because strain-rates accommodated by fault/shear-zone structures are the key to understanding the viscosity structure of the crust and seismic hazard. We constrain the timing and amplitude of slip-rate fluctuations that occurred on three active normal faults in central Italy over a time period of 20–30 kyrs, using in situ36Cl cosmogenic dating of fault planes. We identify five periods of rapid slip on individual faults lasting a few millennia, separated time periods of up to 10 millennia with low or zero slip-rate. The rapid slip pulses migrated across the strike between the faults in two waves from SW to NE. We replicate this migration with a model where rapid slip induces changes in differential stress that drive changes in strain-rate on viscous shear zones that drive slip-rate variability on overlying brittle faults. Earthquakes increase the differential stress and strain-rate on underlying shear zones, which in turn accumulate strain, re-loading stress onto the overlying brittle fault. This positive feedback produces high strain-rate episodes containing several large magnitude surface faulting earthquakes (earthquake clusters), but also reduce the differential stress on the viscous portions of neighbouring fault/shear-zones slowing the occurrence of large-magnitude surface faulting earthquakes (earthquake anticlusters). Shear-zones on faults experiencing anticlusters continue to accumulate viscous strain at a lowered rate, and eventually this loads the overlying brittle fault to failure, initiating a period of rapid slip through the positive feedback process described above, and inducing lowered strain-rates onto neighbouring fault/shear-zones. We show that these patterns of differential stress change can replicate the measured earthquake clustering implied by the 36Cl data. The stress changes are related to the fault geometry in terms of distance and azimuth from the slipping structure, implying that (a) strain-rate and viscosity fluctuations for studies of continental rheology, and (b) slip-rates for seismic hazard purposes are to an extent predictable given knowledge of the fault system geometry.

Kinematic-structural modeling of hybrid fold-thrust belt systems: Insights from the Southern Patagonian Andes

This paper focuses on the analysis of the Southern Patagonian fold-thrust belt at ca. 50° SL, employing a kinematic-structural approach that makes it possible to define its step-by-step structural evolution. Seismic interpretation is combined with outcrop structural data to provide the basis for an integrated evolutionary model including a Jurassic extensional stage, followed by contraction events since the Late Cretaceous. Our interpretation and results reveal a hybrid fold-thrust belt system, with a high decoupling between basement structures and the sedimentary cover. The basement is deformed by tectonic inversion of the Jurassic rift fault system and duplex stacking geometries, while the sedimentary cover is folded by the action of predominantly west-vergent low-angle faults. The Jurassic rifting model identifies listric faults with detachment depths ranging from −6000 to −6500 m.a.s.l. and 11 %–14 % magnitude extension. Shortening calculations yield values of less than 6 %. Growth strata and thickness variations in Cretaceous units suggest that part of the shortening occurred during the Coniacian/Santonian. Overall, this research contributes to the knowledge of fold-thrust belts, highlighting the importance of doing a comprehensive structural modeling including previous deformational stages, and describing steps to provide reliable measurements of deformation.

Geometry and evolution of polygonal fault systems under a regionally anisotropic stress field: Insights from 3D seismic analysis of the Qiongdongnan Basin, NW South China Sea

AbstractPolygonal fault systems (PFS) are developed in many sedimentary basins, and their formation, growth, and ultimate geometry have been widely studied. The geometry and growth of PFS forming under the influence of regionally anisotropic stresses, however, are poorly understood, despite the fact these structures may serve as key paleo‐stress indicators that can help reconstruct the tectonic and stress history of their host basins. We here use high‐quality 3D seismic reflection data and quantitative fault analysis to determine the geometry and evolution of a PFS in the Qiongdongnan Basin (NW South China Sea), and its possible relationship with the geological and stress history of the basin. The PFS is dominated by two intersecting NNW‐to‐N‐ and E‐striking fault sets, which initiated in the Early Miocene. The dominant fault strike at the structural level at which the faults nucleated and where strain is greatest (i.e., Lower Miocene) is close to NW–SE. However, at the top and bottom of the PFS tier faults strike NNW–SSE, thereby defining a very slight vertical, clockwise rotation of strike. Based on the observation that the host rock is flat‐lying, it is unlikely that basin‐tilting perturbed (i.e., δ2 ≠ δ3) the otherwise radially isotropic stress field that typically characterize PFS. Likewise, diapirs that punctuate the host rock and that are spatially related to the PFS appear not to control fault geometry. We instead infer that the PFS geometry reflects a combination of local isotropic and regional, extension‐related tectonics stress affecting the Qiongdongnan Basin during the Early Oligocene to Middle Miocene. Regional studies suggest that during this time, extensional stresses in eastern Qiongdongnan Basin rotated clockwise from roughly NNW to N; we noticed the rotation of strike of the PFS, within which the vertical change in fault strike being relatively minor. Our study determines the timing of polygonal fault growth within the Qiongdongnan Basin and the associated geometry, highlighting the key role played by regional and local stresses.

Reflection seismic profiling along the Longitudinal Valley Basin, Eastern Taiwan: In search of the Central Range fault

A thorough understanding of the internal structures of the Longitudinal Valley is crucial for comprehending the architecture of the suture zone between the Eurasian and Philippine Sea plates. Here, we present the findings of eight new reflection seismic profiles spanning north to south of the Longitudinal Valley. Our primary objective is to investigate the internal sedimentary and tectonic features of the Longitudinal Valley Basin, the nature of the contact between it and the metamorphic basement rocks to the west, and the geometry of the Longitudinal Valley fault system along its eastern flank. A significant outcome of our research is that there is minimal internal deformation within the Longitudinal Valley Basin, with sedimentary features prevailing. The presence of inclined reflections suggests the presence of fan deltas and therefore a marine or lacustrine environment in the Longitudinal Valley. The sedimentary characteristics indicate that the source region for the sediment was predominately the Central Range, but that locally the Coastal Range also supplied sediment. Furthermore, at least in the shallow subsurface (1 to 2 km), a west-dipping Central Range fault does not exist along the western side of the Longitudinal Valley. The Longitudinal Valley fault system does not affect the middle of the basin, instead it is located along the eastern flank, adjoining the topography of the Coastal Range.

Influence of Fault System Geometry and Slip Rates on the Relative Role of Coseismic and Interseismic Stresses on Earthquake Triggering and Recurrence Variability

AbstractWe model Coulomb stress transfer (CST) due to 30 strong earthquakes occurring on normal faults since 1509 CE in Calabria, Italy, including the influence of interseismic loading, and compare the results to existing studies of stress interaction from the Central and Southern Apennines, Italy. The three normal fault systems have different geometries and long‐term slip‐rates. We investigate the extent to which stress transfer can influence the occurrence of future earthquakes and what factors may govern the variability in earthquake recurrence in different fault systems. The Calabrian, Central Apennines, and Southern Apennines fault systems have 91%, 73%, and 70% of faults with mean positive cumulative CST in the time considered; this is due to fewer faults across strike, more across strike stress reductions, and greater along‐strike spacing in the three regions respectively. In regions with close along strike spacing or few faults across strike, such as Calabria and Southern Apennines, the stress loading history is mostly dominated by interseismic loading and most faults are positively stressed before an earthquake occur on them (96% of all faults that ruptured in Calabria; 94% of faults in Southern Apennines), and some of the strongest earthquakes occur on faults with the highest mean cumulative stress of all faults prior to the earthquake. In the Central Apennines, where across strike interactions are the predominant process, 79% of earthquakes occur on faults positively stressed. The results highlight that fault system geometry plays a central role in characterizing the stress evolution associated with earthquake recurrence.

Dynamic Rupture Models, Fault Interaction and Ground Motion Simulations for the Segmented Húsavík‐Flatey Fault Zone, Northern Iceland

AbstractThe Húsavík‐Flatey Fault Zone (HFFZ) is the largest strike‐slip fault in Iceland and poses a high seismic risk to coastal communities. To investigate physics‐based constraints on earthquake hazards, we construct three fault system models of varying geometric complexity and model 79 3‐D multi‐fault dynamic rupture scenarios in the HFFZ. By assuming a simple regional prestress and varying hypocenter locations, we analyze the rupture dynamics, fault interactions, and the associated ground motions up to 2.5 Hz. All models account for regional seismotectonics, topo‐bathymetry, 3‐D subsurface velocity, viscoelastic attenuation, and off‐fault plasticity, and we explore the effect of fault roughness. The rupture scenarios obey earthquake scaling relations and predict magnitudes comparable to those of historical events. We show how fault system geometry and segmentation, hypocenter location, and prestress can affect the potential for rupture cascading, leading to varying slip distributions across different portions of the fault system. Our earthquake scenarios yield spatially heterogeneous near‐field ground motions modulated by geometric complexities, topography, and rupture directivity, particularly in the near‐field. The average ground motion attenuation characteristics of dynamic rupture scenarios of comparable magnitudes and mean stress drop are independent of variations in source complexity, magnitude‐consistent and in good agreement with the latest regional empirical ground motion models. However, physics‐based ground motion variability changes considerably with fault‐distance and increases for unilateral compared to bilateral ruptures. Systematic variations in physics‐based near‐fault ground motions provide important insights into the mechanics and potential earthquake hazard of large strike‐slip fault systems, such as the HFFZ.

Geodetic modelling of the 2022Mw 6.6 Menyuan earthquake: insight into the strain-partitioned northern Qilian Shan fault system and implications for regional tectonics and seismic hazards

SUMMARYStrain partitioning between strike-slip faults in mountains and thrust faults in the foreland is a typical mountain building process to accommodate oblique plate convergence. Studying the geometry and movement of such strain-partitioned fault systems is key to understanding the mountain building process and related seismic hazards. The 2022 Mw 6.6 Menyuan earthquake is the largest strike-slip earthquake to have ruptured the northern Qilian Shan fault system in the modern geodetic era. We combined satellite and field observations to determine the fault geometry and coseismic slip distribution in the Menyuan earthquake, and link the distribution of coseismic slip with the pattern of interseismic strain accumulation within the northern Qilian Shan from our geodetic slip model. We find that the Menyuan earthquake ruptured a 25 km-long section of the left-lateral Longlongling Fault between the surface and 7 km depth. The maximum slip was 4 m at 3–4 km depth. Damage to a high-speed railway tunnel recorded a fault offset of 2.7 m at a depth of 200 m compared to 2.5–3.0 m on the surface, suggesting that dispersion of the rupture through unconsolidated shallow sediments was limited, at least at the tunnel site. We also determined the pattern of interseismic deformation prior to the earthquake using Interferometric Synthetic Aperture Radar and Global Navigation Satellite System data. We found the interseismic geodetic data can be explained by the oblique movement of a low-angle décollement beneath the Qilian Shan rather than a strain partitioning fault system. We suggested that the strike-slip faults and foreland thrusts are separated by a creeping décollement, which would act as a barrier to stop the cascading rupture of the strike-slip and thrust fault.

A new 3-D finite-fault model of the Southwest Iceland bookshelf transform zone

SUMMARYThe largest earthquakes in Iceland occur in the South Iceland seismic zone (SISZ) and the Tjörnes fracture zone in the northeast. With the latter being primarily offshore, the seismic risk in Iceland is highest in the relatively densely populated SISZ. Past probabilistic seismic hazard assessment (PSHA) efforts in Iceland have, however, been based on statistical analyses of various historical earthquake catalogues and limited ground motion models (GMMs), all subject to varying types and degrees of uncertainties. Moreover, they relied on simplistic source descriptions and largely ignored that the unique ‘bookshelf’ strike-slip fault system of the SISZ extends along the plate margins towards the west and over the entire Reykjanes Peninsula Oblique Rift (RPOR) zone. Namely, the bookshelf fault system in Southwest Iceland is twice as long as previously thought and it dominates the strain release of transcurrent plate motion in Southwest Iceland, having potentially important implications for PSHA. In this study, therefore, we propose a new 3-D finite-fault model of the Southwest Iceland bookshelf transform zone. The model has been calibrated on the basis of first principles to the rate of transcurrent plate motions across the transform zone and constrained by the salient features of the fault system geometry as reported in the literature. We model the systematic spatial variability of the seismogenic potential along the zone by its provisional subdivision into six distinct zones. The fault system model allows both deterministic and random fault locations, with each realization completely specified in terms of the maximum expected magnitude of each fault, its maximum dimensions and its long-term slip rate. The variability of the model has been estimated through sensitivity analyses of its key parameters. The total seismic moment rates produced by the fault system model are completely consistent with those reported in the literature. The new model allows the derivation of simple but self-consistent zone-specific Gutenberg–Richter (GR) relationships, and the total long-term seismic activity predicted by the new 3-D fault system model effectively explains the historical earthquake catalogue of the SISZ–RPOR transform zone in Southwest Iceland. We are therefore confident that the model can serve as the foundation for future time-independent physics-based PSHA for Southwest Iceland. Moreover, the consistency and versatility of the model allows its application in conventional approaches to PSHA, which has the potential of bridging the gap between physics-based and conventional approaches to PSHA in Southwest Iceland. Such efforts will improve our understanding of the key elements that affect the hazard, thus improving the reliability of hazard estimates, with important practical implications for the optimized assessment of seismic risk.

Imprint of the Continental Strike‐Slip Fault Geometrical Structure in Geophysical Data

AbstractThe geometry of continental fault systems, and more specifically the spatial organization of faults, is a central topic to understand how earthquake ruptures start, propagate, and stop. By exploring the origin of unexpected high frequency emission during earthquakes, Chu et al. (2021), (https://doi.org/10.1029/2021GL095271) show that the most likely source for these emissions is the interaction between nearby misaligned faults. Thus, this result emphasizes the discrete nature of the strike‐slip fault segments at seismogenic crustal scale, adding to a set of evidence for spatially structured fault systems drawn from independent observations in geophysics and geology. This observation should bring some new constrains to earthquake rupture scenario by limiting the range of possible ruptures included in these models.

Deformation styles related to intraplate strike-slip fault systems of the Saharan-Tunisian Southern Atlas (North Africa): New kinematic models

The complex geometry and tectonic evolution of the Saharan-Tunisian Southern Atlas strike-slip fault systems, North Africa, have not been investigated comprehensively to date. This paper proposes new conceptual models to better understand the structural styles along these intraplate fault zones. We use a multidisciplinary approach including field observations, geological mapping, stress analysis and seismic data to investigate the development and interaction of major strike-slip fault segments. The regional fault system is dominated by the E-W trending Biskra, Alima, Orbata, Bouhedma and Chotts basement faults that constitute the Principal Displacement Zones (PDZs).Our analyses and interpretations reveal that each PDZ has a typical dextral strike-slip architecture comprising multi-scale fault segments that are linked through dilational and contractional stepovers/bends. During Mesozoic times, under NE-SW Shmin, the transtensional fault segments of Biskra and Alima formed the boundaries of the Khanguet Sidi Neji-Gafsa (KSG) Basin, which developed as a Z-shaped dextral releasing stepover. During the Cenozoic, the KSG Basin was inverted, under NW-SE SHmax, in response to the Africa-Eurasia convergence. This inversion occurred mainly along basin sidewalls by reactivation of the pre-existing normal-oblique faults as right-lateral transpressional shears. Further east, the E-W dextral strike-slip faults of Orbata and Bouhedma connected via a NE-SW restraining stepover.Finally, all E-W dextral strike-slip fault segments are associated with ENE right-stepping en echelon folds and Riedel shear patterns of different scales, in response to basement wrenching. Currently, most fault zones are still active with deep strike-slip earthquakes.

Active Extension in a Foreland Trapped Between Two Contractional Chains: The South Apulia Fault System (SAFS)

AbstractThe response of continental forelands to subduction and collision is a widely investigated topic in geodynamics. The deformation occurring within a foreland shared by two opposite‐verging chains, however, is uncommon and poorly understood. The Apulia Swell in the southern end of the Adria microplate (Africa‐Europe plate boundary, central Mediterranean Sea) represents one of these cases, as it is the common foreland of the SW verging Albanides‐Hellenides and the NE verging Southern Apennines merging into the SSE verging Calabrian Arc. We investigated the internal deformation of the Apulia Swell using multiscale geophysical data: multichannel seismic profiles recording up to 12‐s two‐way time (TWT) for a consistent image of the upper crust; high‐resolution multichannel seismic profiles, high‐resolution multibeam bathymetry, and CHIRP profiles acquired by R/V OGS Explora to constrain the Quaternary geological record. The results of our analyses characterize the geometry of the South Apulia Fault System (SAFS), a 100‐km‐long and 12‐km‐wide structure attesting an extensional (and possibly transtensional) response of the foreland to the two contractional fronts. The SAFS consists of two NW‐SE right‐stepping master faults and several secondary structures. The SAFS activity spans from the Early Pleistocene through the Holocene, as testified by the bathymetric and high‐resolution seismic data, with long‐term slip rates in the range of 0.2–0.4 mm/yr. Considering the position within an area with few or none other active faults in the surroundings, the dimension, and the activity rates, the SAFS can be a candidate causative fault of the 20 February 1743, M 6.7, earthquake.

Beyond Backslip: Improvement of Earthquake Simulators from New Hybrid Loading Conditions

AbstractA standard approach to loading earthquake simulators involving complex fault system geometries is the backslip method, by which fault‐slip rates are specified and stressing rates giving the specified slip rates are calculated and imposed on the system. This often results in singularities in stressing rate at fault boundaries, and unrealistic hypocenters of events associated with these singularities. We present a new generalized hybrid loading method that combines the ability to drive faults at desired slip rates while loading with more regularized stressing rates, allowing faults to slip in a more natural way. The resulting behavior shows improvement in the depth dependence of seismicity, the distribution of sizes of events, and the depth dependence of slip. We discuss as well the physical implications of the new type of loading.

Fault slip envelope: a new parametric investigation tool for fault slip based on geomechanics and 3-D fault geometry

Abstract. By combining a 3-D boundary element model, frictional slip theory, and fast computation method, we propose a new tool to improve fault slip analysis that allows the user to analyze a very large number of scenarios of stress and fault mechanical property variations through space and time. Using both synthetic and real fault system geometries, we analyze a very large number of numerical simulations (125 000) using a fast iterative method to define for the first time macroscopic rupture envelopes for fault systems, referred to as “fault slip envelopes”. Fault slip envelopes are defined using variable friction, cohesion, and stress state, and their shape is directly related to the fault system 3-D geometry and the friction coefficient on fault surfaces. The obtained fault slip envelopes show that very complex fault geometry implies low and isotropic strength of the fault system compared to geometry having limited fault orientations relative to the remote stresses, providing strong strength anisotropy. This technique is applied to the realistic geological conditions of the Olkiluoto high-level nuclear waste repository (Finland). The model results suggest that the Olkiluoto fault system has a better ability to slip under the present-day Andersonian thrust stress regime than for the strike-slip and normal stress regimes expected in the future due to the probable presence of an ice sheet. This new tool allows the user to quantify the anisotropy of strength of 3-D real fault networks as a function of a wide range of possible geological conditions and mechanical properties. This can be useful to define the most conservative fault slip hazard case or to account for potential uncertainties in the input data for slip. This technique therefore applies to earthquake hazard studies, geological storage, geothermal resources along faults, and fault leaks or seals in geological reservoirs.

Computational Tools for Relaxing the Fault Segmentation in Probabilistic Seismic Hazard Modelling in Complex Fault Systems

Use of faults in seismic hazard models allows us to capture the recurrence of large-magnitude events and therefore improve the reliability of probabilistic seismic hazard assessment (PSHA). In the past decades, fault segmentation provided an important framework for quantifying fault-based PSHA. Recent complex coseismic ruptures (e.g., 2010 Mw 7.1 Canterbury, 2012 Mw 8.6 Sumatra, 2016 Mw 7.8 Kaikōura, 2016 Mw 6.5 central Italy) have shown the need to consider different possible combinations of rupture scenarios in PSHA. Here we present two new methodologies that model rates of ruptures along complex fault systems, one based on a floating rupture approach (FRESH) and another one based on assumed rupture scenarios (SUNFISH). They represent alternatives to a recently proposed approach (SHERIFS), and further step to overcome the segmented and un-segmented approaches commonly used in PSHA in Europe. Differences among SHERIFS, SUNFiSH and FRESH are related to the way slip rate, rupture geometries and magnitude–frequency distributions are modelled. To quantify the differences between these three methodologies, we compared PSHA results based on geometries and slip rates of a fault system located in northeastern Italy, assuming a given maximum magnitude and the same seismic moment rate target. Differences up to 20–30% in the peak ground acceleration at 2% and 10% in 50 years are observed. Finally, we show that the three methodologies are able to solve for the long-term rate of ruptures with resulting PSHA that reflect the fault system geometry and slip rates, without any assumption on segment boundaries. Using fault-based approaches in PSHA requires collecting as much local geological information as possible. Now that multi-fault rupture approaches are available, simplifying assumptions often made to model complex fault systems (uniform slip rate, segmentation hypothesis) are no longer necessary. On the other hand, local data collection should be strongly encouraged to better characterize the actual fault slip rate variability and the complex 3D geometries.

Stress modeling to determine the through-going active fault geometry of the Western North Anatolian Fault, Turkey

The North Anatolian Fault (NAF) is a 1200 km long dextral strike-slip fault which is part of an east-west trending dextral shear zone (NAF system) between the Anatolian and Eurasian plates. The North Anatolian shear zone widens to the west, complicating potential earthquake rupture paths and highlighting the importance of understanding the geometry of active fault systems. In the central portion of the NAF system, just west of the town of Bolu, the NAF bifurcates into the northern and southern strands, which converge, then diverge to border the Marmara Sea. At their convergence east of the Marmara Sea, these two faults are linked through the Mudurnu Valley. The westward continuation of these two fault traces is marked by further complexities in potential active fault geometry, particularly in the Marmara Sea for the northern strand, and towards the Biga Peninsula for the southern strand. Potential active fault geometries for both strands of the NAF are evaluated by comparing stress models of various fault geometries in these regions to a record of focal mechanisms and inferred paleostress from a lineament analysis. For the Marmara region, the best-fit active fault geometry consists of the northern and southern bounding faults of the Marmara basin, as the model representing this geometry better replicated primary stress orientations seen in focal mechanism data and stress field interpretations. In the Biga Peninsula region, the active geometry of the southern strand has the southern fault merging with the northern fault through a linking fault in a narrow topographic valley. This geometry was selected over the other two as it best replicated the maximum horizontal stresses determined from focal mechanism data and a lineament analysis.

Geometry, kinematics, paleostress analysis and tectonic model of the extensional fault system deforming the Plio-Pleistocene reefal limestone in southeastern Dominican Republic

This paper describes a system of extensional faults oriented NW-SE, NNW-SSE, and ENE-WSW responsible for the tectonic relief of semi-grabens in southeastern Dominican Republic. These faults constitute the natural extension of a previously recognized fault system in western Puerto Rico and the Mona Passage. Paleostress analysis carried out on the faults and fractures in the Pliocene limestone results in a minimum NE-SW stress axis, changing to SSW in the faults that affect the Pleistocene limestone. In contrast, the inversion of focal mechanisms of deep, intermediate, and shallow earthquakes provides a mechanism characteristic of pure thrust, oblique thrust, strike-slip, and normal. Extension in the upper part of the Caribbean crust is accompanied by vertical movements of uplift and subsidence that deform the Plio-Pleistocene coral platform. These movements have been modeled as a broken elastic plate flexed to the trench and incorporated in a more general model of necking in the subducted Atlantic slab. Nevertheless, the flexural mechanism alone can hardly explain the regional extension observed, which is better explained as diffuse rifting, maybe due to the continuity at low rate of the counterclockwise rotation of Puerto Rico.